Electroluminescent (EL) display technology, particularly full-color displays, have very serious limitations in their brightness, which depends upon the rate at which the capacitance of the EL cell is driven. The drive rate is limited by 1/(sq.root (RC)), where RC is the time-constant of the drive lines.
In a conventional display, the IC drivers are mounted at the edge of the display panel and drive all of the pixels in a selected row of the panel. As shown in FIG. 1, a plurality of IC drivers 36a-36h are located on the periphery of the display area. Scanner 37 controls the drivers 36a-36h. Select lines 26, driven by drivers 36a-36h, and data lines 30, driven by register 12, provide access to each pixel cell 32. A pixel cell 32 is located at the intersection of each select line 26 and data line 30. To address the display, a row of display data is supplied from shift register 12 while one of the drivers 36a-36h activates a select line 26. The display data activates pixels in the selected row. In this way, the entire display is addressed row by row.
The electrodes of each pixel cell 32 are connected in series, so that the electrode resistance of all the cells in a given row or column are connected in series. This creates a large RC time constant which limits the maximum possible drive rate and hence the brightness of the display. This is particularly bad, when driving a line of indium-tin-oxide thin-film (a transparent electrode). Typically, this film has a sheet resistivity of approximately 5.OMEGA./square. A 10 mil wide 10 inch long line would thus have a resistance of approximately 5000 ohms.
Moreover, energy is required to charge the row and column electrodes to the voltage level needed for operation. This energy is normally lost when a new signal level is applied. Although energy recovery methods are available, these require added components and are sufficiently expensive that they tend not to be employed. The power used to charge the column electrodes is often an order of magnitude greater than that used to excite phosphor emission, and dominates the power dissipation of EL displays. Other parameters being equal, this power is directly proportional to the length of the column electrodes, and can be reduced if the column can be divided into many short electrodes.
One way to increase the drive rate, and hence the brightness of the display, is known as "active driving." Such a scheme, however, is extremely expensive, because it requires two transistors per pixel. As shown in FIG. 2, each EL cell 26 is driven by two transistors, 14 and 20. Data line 18 and select line 16 are used to activate the EL cell 26. A current source 28 provides current that flows through EL cell 26 and transistor 20 to ground. A capacitance is provided at each cell for signal storage. This type of display has the advantage of allowing all rows of a display to be driven simultaneously instead of one row at a time in displays with no storage capacity. However, they are much more complicated and difficult to fabricate than ordinary EL displays, and are prohibitively costly in large sizes. This configuration also requires the use of a large portion of the display area for mounting the driving transistors.
A somewhat less ambitious scheme is to drive small groups of pixels with a single driver. This, however, requires the ability to make contact with the electrodes not just at the edges of the display, but also at multiple points in the middle of it. Conventional EL panels (on a glass substrate) do not have this capability. An even more efficient scheme, is to drive larger groups of cells (or even complete rows or columns) from a single driver, but in parallel, rather than series.